2001 Freightliner FLD120 Highbeam Issues

I would say the switch is probably bad.

 

Clean plugs at headlight or maybe replace them.

 

Shorted wire between the drivers side plug at either headlight and the splice that goes to the passenger side. Start with a test light at the plug(s) work your way back till you have constant power. Replace wire. Or just buy a whole new harness. Kind of expensive, but will save a lot of headaches in the future if you plan on keeping it a while. Every one I’ve seen has been patched up, including mine. Only a matter of time before another short or broken wire causes problems. Harness is plug and play. Runs to Drivers side and another plugs into it runs along the hood over to passenger side.

 

Check the wires where the hood hinges are.
They like to break there where they bend.